The present invention relates generally to a bed tester for molecular sieve oxygen concentrators.
Molecular sieve oxygen concentrators have become increasingly popular for the production of high purity oxygen (up to 95%) because of their simplicity, reduced energy consumption, and low operating costs. Portable units are now widely used to produce medical oxygen for patients requiring oxygen therapy. Molecular sieve oxygen concentrators are also in use aboard military aircraft for the production of an oxygen enriched breathing gas to prevent hypoxia. In addition, future military aircraft will have oxygen breathing systems employing molecular sieve oxygen concentrators. Oxygen concentrators may have from two to six beds filled with molecular sieve. For further background relating to molecular sieve oxygen concentrators and zeolites, see a copending patent application Ser. No. 07/151,383 filed Feb. 2, 1988, now Pat. No. 4,813,979 issued Mar. 21, 1989 to G. W. Miller and C. F. Theis, "Secondary Oxygen Purifier for Molecular Sieve Oxygen Concentrator", and the following papers: D. M. Ruthven, Sec. 1.4 on "Zeolites" in Principles of Adsorption and Adsorption Process, pages 9-16, John Wiley and Sons, New York, N.Y. (1984); G. W. Miller, Dr. K. G. Ikels, and P. A. Lozano, "Chemical Contamination Studies on a Molecular Sieve Oxygen Concentrator (MSOC): Comparison of MG3 and 5AMG Molecular Sieves", Safe Journal, Vol. 16 No. 4 (1986); D. E. W. Vaughan, "The Synthesis and Manufacture of Zeolites", Chemical Engineering Progress, February 1988, pages 25-31; D. M. Ruthven, "Zeolites as Selective Adsorbents, Chemical Engineering Progress, February 1988, pages 42-50; G. W. Miller, K. S. Knaebel, and K. G. Ikels, "Equilibria of Nitrogen, Oxygen, Argon, and Air in Molecular Sieve 5A", AIChE Journal, February 1987, Vol. 33, No. 2, pages 194-201; G. W. Miller, "Adsorption of Nitrogen, Oxygen, Argon, and Ternary Mixtures of These Gases in 13X Molecular Sieve", American Institute of Chemical Engineers Symposium Series, Vol. 83, No. 259, (1987) pages 28-39.
At present, most molecular sieve oxygen concentrators use 16.times.40 mesh type 5AMG or MG3 molecular sieves, having zeolite 5A and 13X crystals, respectively. The crystal structure has voids in the form of .alpha. cages and .beta. cages, as described in the above papers. Both nitrogen and oxygen are adsorbed in the large .alpha. cages of these zeolites, however, these crystals have a greater affinity for nitrogen due to its slight molecular polarity. Nitrogen and oxygen do not enter the smaller .beta. cages. Due to its small molecular size and nonpolarity, helium adsorbs in negligible quantities, and hence, enters the entire void volume of the zeolite crystals (.alpha. and .beta. cages).
The concentrator's performance or oxygen enriching ability is directly related to the activity of the molecular sieve. Further, the activity of a molecular sieve bed can be degraded by exposure to certain chemical species (principally water) resulting in a reduction in system performance. There is a need for a means for testing the molecular sieve beds to ensure they meet accepted standards of activity.
In the prior art, there are two methods used to determine the activity of a molecular sieve bed. The first method involves reactivating several samples of molecular sieve which have been removed from the bed. The second method involves determining the bed washout pattern using nitrogen and oxygen. Both methods have limitations and disadvantages which are discussed below.
Using the activation method one must remove several samples (3-5) of molecular sieve from the bed. Each sample must be heated to 350 C. for a period of at least four hours at a pressure of approximately one Torr. Based on the weight change of the sample one can calculate the amount of water removed, and therefore, arrive at a value for the weight percent water contained by the sample. The bed weight percent water is determined by averaging the results for all samples. Because bed activity is generally a function of the weight percent water one can arrive at a value for the activity. The limitations and disadvantages of this method are listed below.
1. The activation method requires disassembly of the molecular sieve bed for the removal of several samples. Disassembly and reassembly can be time consuming and must be performed by a skilled technician to ensure the bed is properly reassembled.
2. This method is labor and time intensive. In general, an activity test using this method would require approximately 6-8 hours per bed.
3. If the samples are not taken randomly, this method can give inaccurate results. These inaccuracies may occur because generally only 1-2% of the molecular sieve in the bed undergoes the test.
A schematic diagram of the apparatus required for the washout pattern technique is shown at FIG. 1 (See K. G. Ikels and C. F. Theis, Aviation, Space, and Environmental Medicine, 56: 33-6, 1985.). Using this technique, the molecular sieve bed is first flushed with oxygen via valves V1a, V3a, and V4a. Confirmation of a thoroughly flushed bed would be a 100% oxygen signal at a mass spectrometer 1a. The gas flow is then switched from oxygen to nitrogen via a valve V3a, and the oxygen washout pattern is recorded on a strip chart recorder 2a. A waveform of the nitrogen front exiting the bed is recorded and used to determine the activity of the molecular sieve. The lower the activity of the molecular sieve in the bed the shorter the time required for the nitrogen front to appear. Because washout time is a function of bed activity one can arrive at a value for the bed activity, if one has defined this relationship for the particular bed under test. The limitations and disadvantages of this technique are presented below.
1. Ideally this technique requires a mass spectrometer to analyze the concentration of nitrogen and oxygen in the flow. The cost of this unit is approximately $45,000-60,000. Hence, the cost of an apparatus for testing bed activity based on the washout pattern technique would be expensive.
2. The possibility of obtaining inaccurate values for the bed activity is likely due to the dynamic nature of the washout pattern technique. The results are highly dependent on:
a. The pressure upstream of valve V4a. PA1 b. The steady-state flow setting. PA1 c. The geometry of the particular bed under test. PA1 d. The atmospheric pressure. PA1 e. The diameter of the piping. PA1 f. The response time of the mass spectrometer (if a unit other than a Perkin-Elmer MGA-1100 is employed).
Hence, reproducibility of the data between two apparatuses could be a problem.
3. Use of this technique would require the user to establish a relationship between the washout pattern and activity for each type of molecular sieve bed tested. This relationship would have to be accomplished by a skilled technician.
4. The washout pattern technique also requires a skilled technician to interpret the washout patterns.
United States patents of interest include No. 4,725,293 to Gunderson, which relates to automatic control for a pressure swing adsorption system which fractionalizes air to recover a high purity component. This patent discloses a preferred embodiment in which comparator-controllers are implemented by a microprocessor based programmable controller using software provided therewith which includes Proportional-Integral and Derivative (P-I-D) control algorithms. See, for example, col. 8, line 44 et seq., col. 12, line 4 et seq. and appendix A.
Pat. No. 4,648,888 to Rowland relates to an oxygen concentrator and discloses a controller having a microprocessor which may be programmed to change the sieve bed and/or surge tank charge times to maintain desirable oxygen conentrations in the product gas. Similarly see Pat. No. 4,561,287 to Rowland.
Pat. No. 4,627,860 to Rowland relates to an oxygen concentrator and test apparatus having means for selecting any of the functions monitored by the microprocessor. The test apparatus is connected to the concentrator and displays the selected monitored functions for diagnosing performance levels and component problems or failures. Pat. No. 4,404,005 to Hamlin et al relates to a breathable gas supply for aircrew in a pressurized cabin, comprising a control system based upon a microprocessor which can incorporate a self-test facility. Pat. No. 4,272,265 to Snyder describes apparatus for generating oxygen by the pressure swing method. The apparatus is comprised of a plurality of vessels each having a molecular sieve bed.